In the field of medicine, an intravenous (IV) line is often used to convey a flow of a medicinal liquid into a patient's body. A reservoir containing the medicinal liquid is coupled to a proximal end of the IV line and the distal end is coupled to a large vein of the patient. Also, an IV pump is typically employed with the IV line to precisely control the amount of medicinal liquid delivered to the patient over time. Both peristaltic and disposable cassette pumps are commonly used for this purpose. Most IV pumps employ a sensor to detect the presence of air bubbles in the medicinal liquid carried by the IV line to the patient. If the medicinal fluid carries a large air bubble or a series of smaller air bubbles that can combine within the blood stream, the patient may be exposed to a health risk, since the bubble(s) may produce a life threatening air embolism within the patient's cardiovascular system.
A common problem associated with IV pumps is determining the amount of air in the liquid flowing through the IV line. Also, air bubbles that mix with the medicinal liquid reduce the amount of the medicinal liquid delivered to the patient. Typically, the volume of liquid actually delivered to the patient is determined by measuring the liquid-to-air ratio over a predetermined number of IV pumping cycles. Mechanical failures of a pumping cassette in an IV pump and/or a relatively slow liquid flow are frequently the source of air in the IV line.
In the prior art, an air bubble sensor is usually disposed at a fixed position in a housing of an IV pump. A typical prior art air bubble sensor includes two piezoelectric crystals that are mounted on each side of a slot adapted for gripping a portion of an IV line (tubing). The tubing is forced into the slot so that it is held in close association with the inner surfaces of each side of the slot. In some prior art designs, the IV pump includes an access door that is opened to enable the user to force the tubing into the slot. However, the access door increases the number of parts and the cost of manufacturing an IV pump. Also, forcing the tubing into the slot and operating (opening/closing) the access door increases the likelihood of damage to the tubing. Moreover, since the slot is specifically sized for a particular diameter and type of tubing, medical personnel must stock several different pump models (each with housings having a different slot size) to accommodate the various types and sizes of tubing that may be used. Also, tubing sets with the same external diameter, but having different internal diameters exhibit different stiffness characteristics and may require air bubble sensors specifically designed to accommodate tubing having a specific range of stiffness. Variations in the stiffness due to the use of different compositions of material can also cause problems when forcing the tubing into the slot of conventional air bubble sensors.
In a typical air bubble sensor used on an IV pump, one of two piezoelectric crystals (a transmitter) is excited with an electrical signal at the resonant frequency of the crystal to produce an ultrasonic sound wave, which is directed transversely through the IV line towards the other piezoelectric crystal (a receiver), which is disposed on the opposite side of the IV line. The receiver crystal resonates at approximately the same frequency as the transmitter crystal, and in response to the ultrasonic sound waves that it receives, the receiver produces a corresponding electrical signal that is proportional to the amplitude of the sensed ultrasonic waves. Since it is well known that the transmission of ultrasonic sound waves through a liquid is substantially greater than through a gas, any gaseous (air) bubbles entrained in the liquid flowing through the IV line at the point between the transmitter crystal and the receiver crystal will attenuate the ultrasonic sound waves in proportion to the size and density of the bubbles. Thus, a strong electrical signal produced by the receiver crystal indicates that only a liquid is flowing through the portion of the tubing disposed between the transmitter and receiver crystals, while a weak or missing signal indicates the presence of a gas.
Each change in the magnitude of the ultrasonic sound waves received by the receiver crystal causes a corresponding change in the electrical signal that it produces. Usually, a controller is employed to monitor the electrical signal produced by the receiver crystal, for detecting the presence of air bubbles in the medicinal liquid. The controller generates an alarm and/or stops the IV pump when it detects an air bubble larger than a predetermined maximum or too many relatively smaller gas bubbles passing between the transmitter and receiver crystals over a predetermined time period. However, prior art controllers are susceptible to error when the outer surface of the portion of the tubing disposed between the transmitter and receiver crystals is contaminated with a liquid, i.e., if the tubing is wet. The water on the outer surface of the tubing conveys the ultrasound signal between the transmitter and the receiver crystals, causing a false indication of liquid in the line when air bubbles are actually present.
Based on the foregoing discussion, it will be apparent that a more convenient technique for engaging an IV line with an air bubble sensor would be desirable. Ideally, it should not be necessary for a user to force the IV tubing between the transmitter and receiver crystals of the sensor. Different pumps should not be required to accommodate different size IV tubing in the air bubble sensor slot. There should be no need to open an access door in order to facilitate engaging the IV line with the air bubble sensor. Furthermore, the accuracy of an air bubble sensor should not be affected by the presence of liquid on the exterior surface of the line between the transmitter and receiver crystals, since ambulatory IV pumps that may be worn by a patient while bathing or showering will be exposed to moisture that might cause such errors and fail to indicate a potentially hazardous amount of air in the IV line. Since the prior art air bubble sensors have not properly addressed these problems, it will be apparent that there is a need for a novel air bubble sensor that does.